A known phase retarder is, for example, a thin plate made of inorganic material such as calcite, mica or quartz or a stretched film made of polymer having a high specific birefringent index. The phase retarder may be a quarter-wave plate (hereinafter, referred to as “λ/4 plate”), which converts linearly polarized light into circularly polarized light, or a half-wave plate (hereinafter, referred to as “λ/2 plate”), which turns the plane of polarizing oscillation of linearly polarized light by 90°. These conventional phase retarders can delay the phase of mono-chromatic light by λ/4 or λ/2. They, however, convert white light, which is an associated wave composed of mixture of rays in visible wavelength region, into colored polarized light because the polarization depends on the wavelength and hence because the polarization is distributed over the wavelength region. This is essentially caused by the fact that the phase retarder is made of material having characteristics to give wavelength distribution of the phase retardation.
In order to solve the above problem, various wide-ranged phase retarders giving even phase retardation within wide wavelength regions have been studied. For example, Japanese Patent Provisional Publication No. 10(1998)-68816 discloses a phase retarder prepared by laminating a quarter-wave plate, which gives birefringent light having the phase retardation of λ/4, and a half-wave plate, which gives birefringent light having the phase retardation of λ/2, so that their optical axes may be crossed. Further, Japanese Patent Provisional Publication No. 10(1998)-90521 discloses another combined phase retarder prepared by laminating at least two phase-retarders having optical phase retardation values of 160 to 320 nm so that their slow axes may be neither parallel nor perpendicular to each other.
In producing the above phase retarders, however, it is necessary to perform a complicated procedure in which two polymer films are placed with their optical orientations (such as optical axes and slow axes) controlled. The optical orientation of polymer film generally corresponds to the lateral or longitudinal direction of the film in the form of a sheet or a roll. Accordingly, it is difficult to industrially mass-produce a polymer film having the optical axis or slow axis in an oblique direction of the sheet or roll. In addition, the optical orientations of two polymer films must be positioned so that they may be neither parallel nor perpendicular to each other. The process for producing the above phase retarders, therefore, must include the steps of: cutting two polymer films obliquely at a predetermined angle to obtain chips, and laminating the chips. The step of laminating the chips is so difficult that the axes are often improperly positioned and accordingly that the resultant retarder often has poor quality. As a result, the production yield is lowered and hence the production cost increases. Further, in the step, the chips are often contaminated to impair the quality of resultant retarder. Furthermore, it is also difficult to control strictly the optical phase retardation values of the polymer films, and hence the resultant retarder is liable to deteriorate.
Japanese Patent Provisional Publication No. 11(1999)-52131 discloses yet another phase retarder of laminate type giving a wavelength distribution value α of less than 1 and having a laminated structure in which birefringent media A and B are layered so that their slow axes may be perpendicular to each other. At least one of the birefringent media A and B comprises a liquid crystal compound whose molecules are oriented in homogeneous alignment. In the disclosed retarder, the birefringent media A and B satisfy the conditions of:αA<αB and RA>RBin which αA and αB are wavelength distribution values of birefringent index Δn (α=Δn(450 nm)/Δn(650 nm)) in the birefringent media A and B, respectively; and RA and RB are phase retardations of the media A and B, respectively. Japanese Patent Provisional Publication No. 2000-284126 discloses still another phase retarder comprising two optically anisotropic layers. One of the optically anisotropic layers gives a retardation value of 210 to 300 nm at 550 nm, and the other gives a retardation value of 115 to 150 nm. In addition, one of the optically anisotropic layers is a polymer film, and the other comprises liquid crystal molecules. Japanese Patent Provisional Publication No. 2001-4837 discloses still yet another phase retarder comprising a belt-shaped transparent support and thereon-provided first and second optically anisotropic layers. The first optically anisotropic layer comprises liquid crystal molecules, and gives essentially the phase retardation of π. On the other hand, the second optically anisotropic layer comprises liquid crystal molecules, and gives essentially the phase retardation of π˜□. The first and second optically anisotropic layers are laminated so that the slow axis in plane of the first anisotropic layer may be essentially at 75° to the longitudinal direction of the support and that the slow axis in plane of the second anisotropic layer may be essentially at 15° to that of the first anisotropic layer. Those disclosed phase retarders are laminates comprising two or more layered birefringent media, and are proposed as wide-ranged λ/4 plates.
Even in producing any of the above phase retarders, although at least one birefringent medium comprises liquid crystal molecules oriented in homogeneous alignment, it is still necessary to laminate the two birefringent media with their slow axes positioned perpendicularly or at a predetermined angle. Accordingly, in the production process, a complicated procedure is still indispensable.
In order to solve this problem, PCT Publication No. 00/2675 discloses a process for producing a wide-ranged λ/4 plate comprising not layered phase retarders but only one phase retarder.
In the disclosed process, a film of a copolymer is uniaxially stretched. The copolymer is derived from monomer units having positive refractive anisotropy and monomer units having negative birefringence. Since the thus-stretched polymer film gives inverse wavelength distribution, a wide-ranged λ/4 plate can be constituted of one phase retarding film. This λ/4 plate can solve the above problem, but its thickness is no less than 100 μm, which is thicker than a phase retarder comprising liquid crystal. Meanwhile, it has been recently wanted to thin a phase retarder installed in a liquid crystal display of reflection type. In view of thickness, therefore, the phase retarder of laminate type is required to be improved.
For the purpose of producing a thin wide-ranged λ/4 plate in a simple process, Japanese Patent Provisional Publication No. 2002-267838 proposes a liquid crystal composition giving inverse wavelength distribution. In preparing the disclosed composition, rod-like crystal molecules are mixed with other molecules capable of orienting themselves perpendicularly to the long axes of the rod-like crystal molecules so that the composition can give inverse wavelength distribution. However, since the molecules mixed with the rod-like crystal molecules are not liquid crystal, the mixture loses characters of liquid crystal when the mixing ratio increases. Accordingly, the wavelength distribution is difficult to control. In other words, it is very difficult to realize inverse wavelength distribution similar to the ideal curve of wide-ranged λ/4 plate. It has been, therefore, desired to develop a technique by which a thin phase retarder, such as a wide-ranged λ/4 plate, giving inverse wavelength distribution can be easily prepared in a simple process.